Sentinel Clot in a Basilar Perforator Aneurysm – Allusion to an Elusive Bleeder

Case Description A 53-year-old woman with a sudden onset of severe headache reported with a CT head done elsewhere showing Fisher-4 subarachnoid hemorrhage in the posterior fossa without ventriculomegaly [Figure 1a]. CT-Angiography done at that time revealed no obvious abnormality [Figure 1b and c]. Presenting CT-head at our center on day 18 of ictus revealed a tiny hyperdense focus on the posterior aspect of the basilar top in the prepontine cistern, suggestive of sentinel clot [Figure 1d]. Catheter angiography [Figure 1e] with 3D-rotational angio [Figure 1f] showed a tiny perforator aneurysm at the same site. It was aptly managed by flow-diverter placement from the P1 segment of the posterior cerebral artery (PCA) to the basilar trunk [Figure 2], resulting in effective therapy.Figure 1: Noncontrast CT in axial section (a) showing subarachnoid hemorrhage in the posterior fossa basal cisterns. CT angiography images in sagittal (b) and coronal (c) sections showing the basilar artery with no obvious aneurysm. Noncontrast CT in axial section (d) showing a tiny hyperdense focus (white arrow) in the posterior aspect of the basilar top. Digital subtraction angio images of basilar artery (e) and 3D rotational angiogram (f) in lateral views showing a tiny basilar artery perforator aneurysm at the same location as the sentinel clot on noncontrast CT (arrows in e and f)Figure 2: Digital Subtraction Angiography smart mask images (a and b) showing flow-diverter stenting in anteroposterior and lateral views across the basilar artery perforator aneurysm. Fluoroscopy images in coronal (c) and maximum intensity projection of XperCT (d) images showing optimal placement of the flow-diverter stent (arrows in c and d)Basilar artery perforator aneurysms (BAPA) are rare causes of subarachnoid hemorrhages, which are often occult on angiography. They are small (mean < 2.5 mm in size), often seen only in delayed arterial phases due to slow flow or thrombosis within the aneurysmal sac.[1,2] Susceptibility weighted images can show occult BAPA, due to blood-smearing therein.[3] Magnetic resonance vessel wall imaging is also proven to be effective in diagnosing rupture point of occult and ambiguous aneurysms.[4] Notably, a noncontrast CT can also identify BAPAs, as demonstrated in our case, circumventing the need for sedation or prolonged MRI acquisition times. Digital subtraction angiography (DSA) with 3D rotational angiography should be performed as a gold standard diagnosis in the acute stage itself for early diagnosis. Sometimes initial DSA may not yield diagnosis due to vasospasm. High index of suspicion must be maintained till interval DSA, especially when clinical/imaging pattern of bleed are not corroborating with perimesencephalic nonaneurysmal hemorrhage. BAPA are classified based on their anatomical relation to the perforator origin. Type-I aneurysms arise from basilar trunk adjacent to but not involving the perforator (as in our case). Type-II includes those involving the perforator artery origin. Type-III are the fusiform aneurysms, entirely within the perforator vessel.[2] Endovascular flow diversion is increasingly being accepted as the effective long lasting therapy with accepted risk of the ischemia within the involved perforator artery territory.[5] Abbreviations Computed Tomography- CT Digital subtraction angiography- DSA Middle cerebral artery- MCA Basilar artery perforator aneurysms- BAPA Declaration of patient consent The authors certify that they have obtained all appropriate patient consent forms. In the form, the patient(s) has/have given his/her/their consent for his/her/their images and other clinical information to be reported in the journal. The patients understand that their names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed. Financial support and sponsorship Nil. Conflicts of interest There are no conflicts of interest.